1. Technical Field of the Invention
The present invention relates generally to the mobile communications field and, in particular, to a method that compensates for channelization code limitation during a slotted mode operation in a cellular communication system.
2. Description of Related Art
In existing spread spectrum or Code Division Multiple Access (CDMA) systems, soft intra-frequency handovers are normally used to maintain communications. To perform such soft intra-frequency handovers, a mobile station commences communications with a new base station on the same CDMA frequency assignment, before terminating communications with the old base station. However, in the next generation cellular systems (including, for example, the Wide-Band CDMA or WCDMA systems), the use of inter-frequency handovers (handovers between frequencies) will be essential. As such, handovers between frequencies are needed in CDMA and all other types of cellular systems to support a number of functions. For example, handovers between frequencies are used to support hot-spot scenarios (where a cell uses more carriers than the surrounding cells), hierarchical cell structures (where macro, micro and pico layers are on different frequencies), handovers between different operators, and handovers to other systems (e.g., to the Global System for Mobile Communications or GSM, residing in other frequency bands). The key to providing adequate support of inter-frequency handovers is to provide efficient support of the inter-frequency measurements made. As such, in order to support mobile station inter-frequency measurements in spread spectrum or CDMA systems, a downlink slotted mode of operation has been specified.
In the ARIB, ETSI and TIA specifications for CDMA systems, a combination of channelization codes and scrambling codes are used to separate different physical channels. The data to be transmitted is first spread using the channelization code, and then scrambled using a scrambling code. Typically, a base station will use only one scrambling code for all physical channels, but additional scrambling codes can be used to increase the number of available codes (e.g., to avoid a shortage of codes when introducing adaptive antennas to boost capacity). A method for generating multiple code sets using multiple scrambling codes is disclosed in Swedish Application No. PCT SE 98/01541.
FIG. 1 is a diagram of a channelization code tree (channelization codes are best described by a tree structure). The tree at the upper left in FIG. 1 illustrates the tree construction principle for channelization codes. As shown, codes on the same level in the tree (e.g., 1,1 and 1,xe2x88x921) are orthogonal to one another and have the same spreading factor. If a physical channel is spread with one code in the tree, and another physical channel is spread with another code, which is not on a branch underlying the first code, or on the path from the first code to the root of the tree, the spread physical channels will be orthogonal. Every physical channel is allocated a spreading code from the tree, with spreading factors that match the respective data rates. Subsequent to the channelization process, a scrambling code is applied to the spread data.
In order to support seamless inter-frequency handovers, it must be possible to make inter-frequency measurements on other frequencies without disturbing the normal data flow. Since the user equipment receives the downlink signal continuously, there is no time to carry out measurements on other frequencies using an ordinary receiver. A second receiver can be used to make measurements on other frequencies. However, in order to enable single-receiver terminals to make inter-frequency measurements, a downlink slotted mode has been specified for CDMA systems, in both the ARIB and ETSI technical specifications.
When a base station is operating in the downlink slotted mode, the base station decreases the processing gain of the connection, either by increasing the channel coding rate or reducing the spreading factor by two. A 10 ms data frame can then be transmitted in less than 10 ms, as illustrated by the downlink slotted mode transmission diagram shown in FIG. 2 (with a reduced spreading factor solution shown). As such, the transmission is accomplished with higher power than normal, in order to compensate for the decreased processing gain. Using this approach, an idle period of up to 5 ms is created during which no data is to be received by the user equipment. This idle period can be used to tune the receiver to other frequencies, and signal strength measurements can be performed on those frequencies.
Commonly-assigned U.S. Pat. No. 5,553,014 discloses the use of a slotted mode operation through change of a spreading factor. Commonly-assigned U.S. patent application Ser. No. 636,646 discloses the use of an increased channel coding rate. Finally, commonly-assigned U.S. patent application Ser. No. 636,648 discloses a slotted mode operation using multi-code transmissions.
A significant problem exists with the existing slotted mode approaches. In general, the existing increased channel coding rate solutions will always need a fallback solution using a lower spreading factor, because the rate cannot be increased above a certain limit where quality degradation sets in. For example, if xc2xd rate coding is used, then increasing that rate even more during a slotted mode operation will be difficult. Consequently, it can be concluded that a mode in which a lower spreading factor is used will be needed for a slotted mode operation. As such, this requirement has been identified for standardization in the ETSI technical specification.
When the spreading factor is changed during the slotted frames, it could lead to problems with channelization code shortages. In the downlink, all users share the same set of channelization codes. For example, there are 128 available codes of length 128, which means that 128 channels can be carried simultaneously with a spreading factor of 128. As such, if one channel requires a spreading factor of 64, this will remove two possible codes of length 128, because these codes will no longer be orthogonal to the code of length 64. If the lowest possible spreading factor is reduced by a factor of two due to slotted mode transmissions, this means that the number of available codes that can be allocated to different channels is halved. Consequently, since the code resources were already limited from the beginning, the result of allocating shorter codes for use in a slotted mode operation can be a code limited system. In other words, the system operating in the slotted mode could have its downlink capacity limited by the number of available downlink channelization codes and not by interference. However, as described in detail below, the present invention resolves the above-described problems.
In accordance with the present invention, the downlink channelization code limitation problem encountered in spread spectrum or CDMA cellular systems is resolved by using codes from a different, non-orthogonal code set when operating in the slotted mode. The non-orthogonal code sets can be constructed by using the same channelization code tree, but applying a different scrambling code.
An important technical advantage of the present invention is that channelization code limitations are compensated for while operating in the slotted mode.
Another important technical advantage of the present invention is that the spreading factor can be halved for slotted mode operations in a spread spectrum or CDMA cellular communications system, without reducing the number of channelization codes available.
Still another important technical advantage of the present invention is that seamless inter-frequency measurements can be made for all services in a cellular communications system, regardless of the channel coding rate, etc.
Yet another important technical advantage of the present invention is that it makes complex reallocation of code resources in a slotted mode of operation unnecessary.
Still another important technical advantage of the present invention is that the behavior in the slotted mode operation is deterministic, so the network and user equipment need only agree on when to perform slotted transmission, and not how to perform them.